Safety-limit switch for electric hoists



May 8, 1923. 1,454,827

M. A. BECK SAFETY LIMIT SWITCH FOR snacmxc' mm aneqA'Jan. 12. 1920 Sheets-Sheet 1 vi N i i May 8, 1923. 1,454,827

M. A. BECK SAFETY um-r swn-cn For: ELECTRIC HOISTS Filed Jan. 12. 1920 z'sheets-sheet 2 E S E rvwenibn- Patented May 8, 1923.

NHTE S MATTHIAS A. BECK, OF MILWAUKEE, WISCONSIN, ASSIGNOR TO MILWAUKEE ELEC- TRIO CRANE & MFG. 00., INCORPORATED, OF MILLBROOK, YORK, A CORPORA- TION or new YORK.

SAFETY-LIMIT SWITCH FOR ELECTRIC HOISTS.

Application filed January 12, 1920. Serial No. 350,762.

To all whom it may concern Be it known that I, MATTHIAS A. BECK, a citizen of the United States, residing at Milwaukee, in the county of Mil waukee and State of Wisconsin, have invented certain new and useful Improvements in Safety- Limit Switches for Electric Hoists, of which the following is a specification, reference being had to the accompanying drawing, forming a partthereof.

The main objects of this invention are to automatically open the circuit of an electric crane or hoist and thus prevent accidents and injury to the hoisting apparatus and attendants by overwinding of the hoisting cable or elevation of the load beyond a cer tain limit; to automatically close and .restore the hoisting circuit to operative conditio-n when the load is lowered from the safety limit; to prevent vibration of the crane or hoisting apparatus on "which the limit switch is mounted from releasing'the movable member of the switch; and generally to improve the construction and operation of switches of this class.

The invention consists in the construction,

arrangement and combination of parts as hereinafter particularly described and pointed out in the claims.

In the accompanying drawing like characters designate the same parts in the several figures. v

Figure 1 is a section on the line 11, Fig. 2, of a safety-limit switch embodying the invention; Fig. 2 is a section of the switch on the line 22, Fig. 1; and Fig. 3 is a diagram illustrating the circuit connections of the switch as applied toatypical electric crane or hoist.

Referring to Figs. 1 and 2, the switch comprises a case 1, provided with a removable cover 2, one or more relatively fixed contact members and one or more movable contact members, enclosed by the case.

In the present case a double break switch is shown, having two fixed and two movable contact members. Each fixed contact member comprises a carbon block 4, fastened with an interposed insulating block 5 to one side 50 of the case 1. Each movable contact membercomprises a metal spring 6,fastened to and insulated from a metal plate or support'i', which is pivotally mounted on a The support 7 is formed or provided with an arm 9 connected with the case by a spring 10 wh ch tends to shift the springs 6 out of engagement with the blocks 4. The cylinv di-ical hub of the plate or support 7 through which the pivot pin 8 passes, is formed with a notch and radial shoulder 11 and with a lug or projection 12, having a face radial to the pivot pin. 1

The case 1 is providedwith anv insulating lining 14, around the contact pieces 1 and 6, and between the two fixed contact blocks or pieces 4 an insulating partition 15 is interposed, to from arcing.

A rocker shaft 17 is supported by bearings in the case 2 parallel with and'adjacent to the pivot pin 8 of the movable contact member. On the shaft 17 within the case, is pivotally mounted a pawl or detent .18, adapted by engagement with the shoulder 11 to .releasably hold the contact springs 6 in engagement with the contact blocks 4. At

the end opposite that which engages with the shoulder 11, the pawl or detent 18 is formed with a lug or projection 19, having a face prevent injury to the switch radial to the shaft 17. The hub of the pawl' or detent is formed or provided adjacent to the support 7 with a cam 20, adapted by engagement therewith to open the switch independently of the spring 10, as hereinaf- A spring 23, fastened to the member 22, I

tends to hold the pawl 18 against the hub of the pivoted support-7 and in engagement with the shoulder 11. i

The rocker shaft 17 is provided outside of the case 1 with means adapted to co-' operate with a part of the crane or hoist for releasing the limit switch and stopping the motor when the hook block or sheave of the hoist is lifted to a certain point and for closing the switch and automatically restoring the hoisting circuit of the motor when the hook block or sheave is lowered from such point. For this purpose a weightedor 'gravity arm 25., fastenedto one end of the shaft 17 outside of the case 1, and tending normally to turn the shaft in a "clock ise'direaiona d to hold thelmeinber 22 in en agement with the lug or projection 12, is shown in Figs. 1 and 3.

The contact springs f3 are electrically onnected with each other by a metal plate or bar 27, and the contact blocks or pieces 1.

are connected by wires 28 with a shunt or branch-29 from the hoisting circuit 30 0f the motor, as shown by the diagram, Fig. 3. The shunt or branch 29 includes the coil of a solenoid or magnetic cut-out 32, which is in and controls the circuit 30". Thearmature and field of the hoisting motor are designated inFig. 3, a and 7 respectively.

"The safety-limit switch may be suspended by a bracket or hanger 34:, as' shown in Fig.- 3,. from the trolley (not shown) of the crane, or otherwise mounted on a part of the hoist with the weighted or gravity arm 25 extending ther efrom between the runs of the. chain or cable 0 by which the hook block or sheave h is suspended from the hoisting drum (not shown) of the crane.

In the operation of the safety-limit switch,

i case. the operator should inadvertently al low the hook block it to be lifted above the limit ofsafety, it will by engagement with the army 25, turn the rocker shaft 17 with the member 221 fixed thereon, to the left or counterclockwise, as shown in Fig. 2, and the member 22 by engagement with the projection 19 on the pawl 18, will disengage the pawl. from the shoulder 11, thereby re leasing the pivoted support 7 whereupon the contactsprings 6 will be instantly thrown by the snap-aiction of the spring 10 away from the fixed contact blocks 1, thus automatically. breaking the branch circuit through the coil of the solenoid or magnetic cut-out 32, releasing the switch, which opens the hoisting circuit 30 and stops the motor, and leaving the lowering circuit .of the motorin operative condition.

As the member 22 is turned to the left and disengages the pawl 18 from the shoulder 11.it is moved away from the projection 12 and allows the switch to open. When the pawl 18 is disengaged from the shoulder 11 and'rele'ases the support 7 the projection 12 is brought into engagement with the member 22'which acts as a stop to limit the opening movement of the contact springs 6L When the. hook block it is lowered sufiiciently to release the. arm 25, the weight of the. arm: turns the. rocker shaft 17 .with the member 22, backto the right, or clockwise, as shown in Fig. 2, and the :member 22 engaging. the. projection 12, turns the support 7 against the tension of the spring 10, till thecontact'springs 6 engage with thecontact bloeks.4,?tl1ereby. closing'the switch and restoring the shunt or branch circuit 28 to operative conditionat theilimit switch, and the pawl 18 engages with the shoulder 11, thereby locking the switch closed sun cient play or lost motion is provided between the ends of the member22 andlthe projections 12 and 19 when the switch is closed and the pawl 18 ,is engaged with the shoulder 11, as shown in Fig. 2, to allow for a limited movement of the arm 25 which may result from vibration or shocks of the bridge or part of the crane or hoist to which n the limit switch is attached,-without disenthe pawl or detent-18 from theshoulder 11 and thereby accidentally releasingand permitting the switch to open.

Various changes in the construction-and arrangement of parts of the limitswitch-and; of its clrcuit connections with a crane or gagementwith the other, means constantly.

tending to open the switch, means for re-.

leasably holding the switch closed, and

means adapted to beactuated by the shift ing of a part of a hoist to a certainclimit" to releasesaid movable contact-member,

and when said part recedes from sa1d'hm1t,..

to automatically close the switch.

21in a safety-limit SWItChfOI electric hoists, the combination of contact members one of which is movableinto. and out ofon gagement with the other, 'meansconstantly tending to shift the movable member out of engagement with the other, a detentxfor... releasably holding the movable member. inw

engagement .with the other, and *meahs adapted to be actuated bya movable part .110 i of a hoist when it is shifted to..a certainlimit to disengage the detent and-release the movablecontact member and .toautof matic allyisliiftssaid movable Contact meme. her back into engagement. Withmthe other.

contact memberwhen said movablei part recedes from said limit.

3. In a safety-limit switch for. electric.

hoists .the combination of a relatively-fixed contact member, a pivotally.mounted-"cone tact member movable intoa'nd out of engagement with the fixed member, means-v tending to shift the movable. member away from the. fixed member, a, detent for'releasably holding the movable member in engagement with the fixed. member, anloscil.-'y

adapted to be actuated by a movable part.

of a hoist when it is shifted to a certain limit to disengage the detent and release the movable contact member.

4. In a safety-limit switch for electric hoists the combination of a relatively fixed contact member, a pivotally mounted contact member provided with a shoulder and a projection, means tending to shift the pivoted contact member away from the fixed 10 member, a pivoted detent adapted by engagement with the shoulder on the pivoted contact member to releasably hold said member in engagement with the fixed contact member and provided with a projection, 35 and a rocker member mounted coaxially with the detent and adapted when turned in one direction by engagement with the projection on the detent to disengage the detent and release the pivoted contact member, and when turned in the other direction by engagement with the projection on the pivoted contact member to shift said member into engagement with the fixed contact member. g

85 5. In a safety-limit switch for electric hoists the combination of a relatively fixed contact member, a contact member movable into and out of engagement with the fixed member and provided with a shoulder and a projection, means tending to shift the movable contact member away from the fixed member, a detent provided with a projection and adapted by engagement with the shoulder on the pivoted contact memher to releasably hold said member in engagement with the fixed contact member, and a controlling member movable with lost motion between the projections on the detent and movable contact member and 9 adapted when shifted in one direction to 6. In a safety-limit switch for electric hoists the combination of a relatively fixed contact member, a pivoted contact member provided with a shoulder and a projection, means tending to shift and hold the pivoted member out, of engagement with the fixed member, a pivoted detent adapted by engagement with the shoulder on the pivoted contact member to releasably hold said,

member in engagement with the fixed member and provided with a projection, a rocker member coaxial with the pivot of the detent and adapted to cooperate with the projections on-the detent and pivoted contact member, and a gravity member connected with the rocker member and adapted to. be actuated by a movable part of a hoist when it is lifted to a certain limit to disengage the detent and release the pivoted contact member and when said movable part descends from said limit to shift the pivoted contact member back into engagement with the fixed member.

7. In a safety-limit switch for. electric hoists the combination ofa relatively fixed contactmember, a; pivoted contact member provided with a projection, a pivoted deber away from the fixed'member when the detent is disengaged from the pivoted contact member by the rocker member.v

In witness whereof I hereunto afiix my signature.

MATTHIASA. BECK. 

